The David Rumsey Historical Map Collection from Cartography Associates offers a wealth of cartographic resources to the historian–both for research and for teaching. The collection, which is heavy on the Americas, but includes historical maps from around the world, uses a proprietary software platform to allow web users to view files that are often extremely large (more than 1 gigabyte) in high resolution without sacrificing much in the way of download speed. The entire project is the brainchild of David Rumsey, map collector, entrepreneur, and supporter of education.
Here is an example of a map of Bohemia–first the original size displayed, and then a zoom on central Bohemia which gives you a sense for the quality of the scans.
Several maps can also be displayed on screen at once, allowing easy comparison of maps of the same geography but from different eras or different cartographers. Seen side by side like that, multiple maps can ask students to consider why one cartographer would represent the geography one way, while another cartographer would represent it in a different way?
Finally, although the interface is the clunkiest, there are several maps of North America that can be viewed as GIS applications. These take much longer to load and refresh over my at home DSL connection, but still load fast enough to be fun to play with. The half dozen or so examples of GIS here can be used to show students how GIS can be used to make sense of historical data. If you have connection speed issues, be sure to use the basic browser option.
Best of all, the entire collection is posted under a Creative Commons license, so it is free for use by educators and students alike.